The invention disclosed herein is directed to improvements in reel-to-reel sheet material transfer systems and methods, and more particularly to the determination of the diameter of one or both of the reels or spools (with sheet material wound thereon) in such systems. The sheet material may be in strip form such as ribbon, tape, film, etc. In the particular application disclosed herein, the sheet material is a thermal transfer or thermal donor medium moved past a thermal print head in a thermal printer, which transfers pigment, wax, resin, ink, etc. from the donor medium to a receptor medium such as paper.
In a transport system for transporting a sheet material from a supply reel thereof, past a station at which the sheet material is operated on (e.g., past a thermal print head in a thermal printer, or past a magnetic head which reads audio or video tapes) to a take-up reel on which the sheet material is rewound, it is desirable to apply a selected tension on the sheet material as it is moved past the station. The selected tension may be constant for a wide range of conditions, or different selected tensions may be applied in accordance for different conditions.
For example, thermal printers utilize a thermal transfer or donor medium (hereafter referred to as a "thermal transfer ribbon" or simply as "ribbon") containing pigment, wax, resin, ink, etc. (hereinafter referred to as "ink") which is transferred in a desired pattern to a receptor medium, e.g. paper, by a thermal print head. During printing, thermal elements in the print head contact the thermal transfer ribbon and press the ribbon against the receptor medium which is supported by a platen. By heat and some pressure the print head activates and transfers the ink carried by the ribbon (donor) onto the receptor medium. The ribbon and receptor medium are maintained in contact and heat is applied by the print head for a predetermined minimum "dwell" time sufficient to effect transfer of the ink to the receptor medium. Typically, the thermal transfer ribbon becomes temporarily adhered to the receptor medium during the dwell time as the ink is transferred thereto. The receptor medium is typically continuously moved past the print head at a rate slow enough to permit the print head to heat and press the ribbon against the receptor medium for at least the minimum required dwell time.
The tension imparted to the ribbon by a motor which drives the take-up reel, assisted by movement of the receptor medium with the ribbon pressed against it, unwinds ribbon from the supply reel and rewinds the ribbon on the take-up reel after the ribbon has passed adjacent the print head. Unused ribbon must be unwound from the supply reel, moved past the print head and rewound on the take-up reel at a rate which ensures that no used ribbon portion is adjacent the print head during printing, otherwise portions of the image to be printed will be skipped if there is coincidence between a used portion of the ribbon and the particular thermal elements in the print head that are activated during printing. At the same time, to conserve ribbon, the ribbon should not be unwound and rewound at such a high rate that unused ribbon is rewound on the take-up reel.
The tension on the ribbon also affects the drag on the receptor medium drive system, as well as movement of the ribbon past the print head. Proper tensioning of the ribbon reduces drag of the receptor medium on the drive system therefor, and also establishes the proper peel angle and donor/receptor dwell time. Stated another way, proper tensioning helps offset the braking effect caused by the print head bearing against the receptor medium.
Thus, it would be desirable in such an application to control the tension on the ribbon, and to be able to do so as operating parameters and conditions change, such as changes in the diameters of ribbon wound on the respective reels, changes in the type ribbon used, changes in the type receptor medium used, changes in the printing speed, etc. It is further desirable to accomplish such tension control continuously in real time. To accomplish such tension control, it may not only be necessary to adjust the drive torque to the take-up reel to maintain a constant tension on the ribbon, but also to adjust the torque to change the tension on the ribbon as operating parameters or conditions change.
Prior art mechanical arrangements are not entirely satisfactory for accomplishing sheet material tension control as described above. For example, relatively simple spring-loaded or counter-balanced tension control systems for reel-to-reel sheet material transport systems suffer from the drawback that they do not accurately control sheet material tension as operating conditions change and they often require direct contact of a sensor element with the media to be gauged, while other tension control systems that may be able to accomplish the tension control described above would be relatively complicated or expensive.
It is also desirable in reel-to-reel sheet material transport systems to determine the quantity of sheet material on the supply and take-up reels. For example, in the thermal printer described above, it would be helpful to determine when the transfer ribbon on the supply reel is about to be exhausted so that printing may be stopped before the ribbon runs out and a new reel may be loaded into position. Reel diameter determination is frequently performed by visual sighting, or by directly mechanically, electromechanically, or optoelectrically sensing the reel edge location. These techniques are either not accurate, require physical contact with the reel or ribbon, or are expensive to implement.
There is therefore a need for an improved sheet material tension control system and method, as well as for an improved sheet material diameter determining system and method.